Paint Calculator

What does the paint calculator do?

The paint calculator estimates how much paint you need for a room. It works out the wall area from the room size, takes away doors and windows, multiplies by the number of coats and divides by the coverage printed on the paint can. It then adds a little extra for waste and suggests a combination of standard can sizes, so you buy enough paint without a lot left over.

How to estimate paint

1. Choose metric or imperial units.
2. Enter the room length, width and wall height, and choose how many walls you are painting.
3. Enter the number and size of doors and windows to subtract.
4. Set the number of coats and the coverage from the paint label. The defaults are about 10 square metres per litre or 350 square feet per gallon.
5. Tick the ceiling option if you are painting it too, then read the result and the suggested cans.

What the calculator includes

• Wall area from the room perimeter and height, or from two or three walls for a feature wall or partial job.
• Deductions for any number of doors and windows.
• Coats and coverage that you can change to match your product.
• Extra percentage for spills, rollers and touch-ups, ten percent by default.
• Can suggestions in litres (20, 10, 4, 1 and 0.5 L) or gallons (5 gal, 1 gal and quarts), with the option that leaves the least paint over and the option with the fewest cans.

Formula used

Paint needed = (wall area – doors – windows + ceiling) x coats / coverage x (1 + extra). For three walls the tool uses two lengths plus one width, and for two walls one length plus one width, so measure the walls you will actually paint.

Tips for buying paint

Coverage figures on the can assume a smooth, primed surface. New plaster, bare wood, textured walls and a big change from a dark to a light colour soak up more paint, so plan for an extra coat or a primer. Two coats are standard for an even finish. Buy all your paint at the same time so the colour batch matches, and keep a small amount for touch-ups later. Ceilings usually take a different, flat paint, so you may want to calculate them separately.

Example

A room 4 m long, 3.5 m wide and 2.7 m high has 40.5 square metres of wall. Taking away one door of 0.9 x 2.1 m and two windows of 1.2 x 1.2 m leaves about 35.7 square metres. Two coats at 10 square metres per litre need about 7.1 litres, or about 7.9 litres with ten percent extra. The tool suggests two 4 litre cans, which covers the job with a little left over for touch-ups.